Light sensitive photothermographic or heat sensitive film typically includes a thin polymer or paper base coated, generally on one side, with an emulsion of dry silver or other heat sensitive material. Such photothermographic film is normally processed or developed at a temperature generally in the vicinity of 120 degrees centigrade. To produce a high quality image, controlling heat transfer to the photothermographic film during the development process is critical. If heat transfer is not uniform during development, visual artifacts such as non-uniform density and streaking may occur. If heat is transferred too quickly, the base of some types of photothermographic film can expand too rapidly, resulting in expansion wrinkles that can cause visual artifacts in a developed image.
Several processing machines have been developed in efforts to achieve optimal heat transfer to the photothermographic film during processing. One employs a heated drum with multiple rollers around the exterior of the drum's circumference to press the film against the drum. This technique is typically best suited for film having an emulsion coating on only one side, as more heat is generally transferred to the side of the film facing the drum as compared to the side opposite the drum. Another machine slides the photothermographic film over flat heated surfaces in a horizontal path or over plates arranged in a circular path. Still another machine is a flat-path processor having rollers above and below the film to transport the film through the processor.
The processors in each of these machines heats the photothermographic film to a processing temperature and maintains the film at the processing temperature for a set time for optimal development. One processor includes a preheat zone that rapidly heats the film to the development temperature to initiate the development process, and a dwell zone that keeps the film at the development temperature for the set time to complete development.
While such processors are effective at developing photothermographic films prepared using polymeric binders coated from organic solvents, they are not as well-suited for processing newly emerging gelatin-based photothermographic films. These films are coated from aqueous-based solvents, contain heat sensitive materials such as developers, and require a higher development temperature. The moisture content of these aqueous-based emulsions can affect the heat transfer characteristics of the film and, consequently, the quality of images produced during processing. The moisture level of these emulsions is also susceptible to changes depending on the temperature and humidity of the environment in which they are stored and used. Consequently, the moisture level of the emulsion can vary between films. This can result in film-to-film variations in image quality after processing.
It is evident that there is a need for a photothermographic film processor capable of uniformly developing gelatin-based photothermographic film without introducing visual artifacts as described above.